I had this problem with openSuse 11.3 booting. I tried Unetbootin and also tried using initrtud (as a replacement of initrd since some users said this would work) but all in vein.

Finally, after spending couple of nights working on this problem, I got the solution and this is how simple it is:

1. Replaced my USB HDD with USB FDD (for some reason, booting fails with USB HDD)
2. Erased USB FDD (you can Ubuntu Startup disk creator if you wish)
3. Use this command (after su/sudo): dd if=/<iso or .raw file location> of=/dev/sdX bs=8MB

NOTE: /dev/sdX where in sdX, X is where your USB is mapped (check this using 'df -h' command). If you see /dev/sdb (and /dev/sdb1 ...), use /dev/sdb
4. Reboot your laptop/netbook (I used Samsung N148 netbook)

If this does not work for you, then let me know.

Regards,
Rajesh
http://rajeshg.info